This character is a transwoman.

Transsexual people experience a gender identity that is inconsistent or not culturally associated with their assigned sex, and desire to permanently transition to the gender with which they identify- usually seeking medical assistance to help them align their body with their identified sex or gender.

A transwoman is a woman who was assigned male at birth, but who has already made a permanent or partial transition, or have recently started to transition to the opposite sex (this could be before or during the story).

Please note that transwomen and futanari are not the same- transwomen only have one set of genitalia whereas futanari possess both male and female genitalia.